Light Alloys- Aluminium Properties, Uses and Extraction Flashcards
The good properties of aluminium
Low density (1/3 of steel)
Highly resistant to most forms of corrosion
Superb conductor of electricity (along with other intrinsic qualities mean has replaced Cu in many electrical applications)
Non-magnetic and non-combustible (invaluable for electronics or offshore structures)
Non-toxic and impervious (good for food and packaging)
Strong and ductile (along with low density mean extensive use in transport)
Corrosion resistance of aluminium
Metal’s natural coating of Al2O3 provides highly effective barrier to the ravages of air, temperature, moisture and chemical attack making Al a useful construction material
How abundant is Al?
3rd most abundant element in earth’s crust
Atomic number and mass
13 and 26.98
Construction industry
Used for external features like roofing, doors, window frames, gutters.
Also internal features like ceilings, doors, elevators, floor furniture, partition walls, signs, Venetian blinds
Which modes of transport use Al?
High speed trains and ferries, automotive (car body), aerospace.
How does lightweighting of cars affect CO2 emissions?
For every 100kg saved on mass of car this equates to reduction of 9g of CO2 per km.
Recyclability of Al in cars
95% of end-of-life Al in cars can be recycled. EU and Japan require 95% recycling of the total material used in new cars
Secondary weight savings in Al cars
When Al extensively used, secondary weight savings can exceed 50%. These are due to allowing drivetrain, motor and chassis downsizing
How much Al do drivetrain, chassis and suspension and car body generally use?
Drivetrain (engine block, radiator…) 50%.
Chassis and suspension (wheels, steering…) 30%.
Car body (bonnet, doors, bumper…) 20%
Use in packaging
Disposable food containers, drink cans, aerosol cans
Electrical and thermal applications
Pans, motors. Most overhead transmission lines are Al
Other application of Al
Furniture, accessories, art
Where is Al often made?
Where hydroelectric power is cheap
Where does primary Al come from?
Bauxite. Then goes to alumina then Al
Stages of Al extraction
Bauxite is a mixed oxide ore containing about 50% hydrated alumina. Separated from other oxides by dissolution and reprecipitation reaction with NaOH (Bayer process). Then dissolved in a molten salt mixture of cryolite (Na3AlF6) and electrolytically reduced to molten Al (Hall-Heroult process). 4kg bauxite makes 2kg Al2O3 makes 1kg Al.
What is useful about Al for its extraction?
Most metals only dissolve in acid but Al also dissolves in alkali so can be selectively removed
How much power is used to extract Al from bauxite?
70,000kWh consumed per tonne of Al produced from bauxite. Steel requires 13-16,000 kWh/tonne from iron ore to finished product
Bauxite resources
Estimated to be 55-75 billion tons. Africa (33%), Oceania (24%), South America and Caribbean (22%), Asia (15%) and elsewhere (6%)
How is Hall-Heroult process arranged?
Molten mixture of Al2O3 and cryolite (Na3AlF6) along with additives like CaF and AlF3. This is in a graphite-lined tank which is the cathode. Have graphite electrodes (anodes) inside. Liquid Al forms at bottom and exits at bottom (continuous process). Typically have 300 cells connected in series to form a potline.
Reactions and temperatures in Hall-Heroult process
Overall reaction is 2Al2O3 + 3C -> 4Al + 3CO2
Forms bubbles fo CO2 at the anodes. Alumina melts at 2040°C and is poor electrical conductor. Typical melt contains 80-90% cryolite. Continuous process with bath running all the time at 950°C. Largest cells have daily output of 4tonnes
Why does aerospace sector waste so much Al?
Needs large blocks for suitable thermomechanical processing for right grain structure and properties. Wastes around 80% of Al it uses. Aerospace industry relies on Al industry not the other way around
